/*
 * Copyright (c) 2013, SRI International
 * All rights reserved.
 * Licensed under the The BSD 3-Clause License;
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 * 
 * http://opensource.org/licenses/BSD-3-Clause
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 * 
 * Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 * 
 * Neither the name of the aic-expresso nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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package com.sri.ai.grinder.library.number;

import com.google.common.annotations.Beta;
import com.sri.ai.expresso.api.Expression;
import com.sri.ai.expresso.core.DefaultSymbol;
import com.sri.ai.expresso.helper.Expressions;
import com.sri.ai.grinder.api.RewritingProcess;
import com.sri.ai.grinder.core.AbstractRewriter;
import com.sri.ai.util.math.Rational;

/**
 * Implements a rewriter for the exponentiation operation.
 * 
 * @author braz
 *
 */
@Beta
public class Exponentiation extends AbstractRewriter {

	@Override
	public Expression rewriteAfterBookkeeping(Expression expression, RewritingProcess process) {
		if (expression.hasFunctor("^") && expression.numberOfArguments() == 2) {
			Expression base = expression.get(0);
			Expression exponent = expression.get(1);

			if (Expressions.isNumber(exponent)) {
				Rational exponentValue = exponent.rationalValue();
				if (exponentValue.isZero()) {
					return Expressions.ONE;
				}
				if (exponentValue.isOne()) {
					return base;
				}
			}
			
			if (Expressions.isNumber(base)) {
				Rational baseValue = base.rationalValue();
				if (baseValue.isOne()) {
					return Expressions.ONE;
				}
				// we refrain from simplifying 0^x to 0, because x could be 0 itself.
				
				if (Expressions.isNumber(exponent)) {
					Expression result = null;
					try {
						int exponentIntValue = exponent.intValueExact();
						result = DefaultSymbol.createSymbol(baseValue.pow(exponentIntValue));
					}
					catch (ArithmeticException e) {
						// Rational.pow does not work for non-int exponents, so we have no choice but lose precision here.
						double exponentDoubleValue = exponent.doubleValue();
						double baseDoubleValue = base.doubleValue();
						result = DefaultSymbol.createSymbol(Math.pow(baseDoubleValue, exponentDoubleValue));
					}
					return result;
				}
			}
		}
		return expression;
	}
}
